It looks like you're keeping yourself in excellent physical condition! There are various arrythmia meds out there besides beta blockers that might work great for you since you don't have any underlying heart issues. I know people, including myself who have stayed on an anti-arrythmic for a year or longer with no episodes. One on here in particular, claytex, has been on flecainide for over a year and he is 68 and very active also. You might touch base with him. You know, all the Dr.s say this is a progressive disorder and it will only get worse over time and more frequent and longer episodes. I am contemplating an ablation and have an excellent EP lined up but I still have strong reservations about it. It might be your answer...then again,it might not. They don't always work and sometimes it calls for a 2nd or 3rd. At any rate, it's not a piece of cake. There are many threads on this website about a-fib....so enjoy :)
I think you have two things working against you (given you have some biological tendency to AFib).
2) High physical stress/exercise
For those of us who will develop, or have, AFib the two factors above are factors.
I was a "runner" (so to speak, but did run HR in the 160 range) and about the age of 55 I started have heart rhythm problems which were diagnosed as AFib when I was 57. I underwent electrocardioversion with following drugs of Toprol and Propafenone (not both) to hold me in sinus for periods up to 18 months. I returned to running. At age 67 I had some complications (will not describe - see profile if interested) and have tried electrocardioversion as recent as August 2008, it didn't hold.
Sounds to me like you might get (temporary, sorry to say) relief from a higher dose of metoprolol. I was able to take 50 mg of Toprolol when I was closer to your current age and the side effects on me were minimal to none - only thing I noticed was a lower resting HR. I have taken as high as 200 mg and that gives me problems on the side effect side.
I have been involved in competitive cycling and running for over 30 years. My workouts were much like yours with an average hear rate of 155 for 40 minutes or more and periods up to 180. In September of 2008 I started to have SVE and PACs with non sustained A-fib. Tests indicated my heart was normal and I was referred to an Electro-physiologist. I was told I have a preliminary A- fib condition and the EP starting me with drugs that are supposedly mild with minimum side effects . This includes Rythmol SR 225 MG twice per day and Metoprolol Succer 25 MG once per day. The combination of these drugs reduced the frequency of the SVE's but I lost the top end of my power. No matter how hard I pushed I could not get my heart rate over around 130. I think your body adapts somewhat and becomes more efficient at lower heart rates as aerobic power did not decline as much as one would suspect with such a cap on max heart rate. However, I did loose around 10% to 15% which is unacceptable for competitive endurance sports.
In July of 2009 I went through the ablation procedure. My rational was that the condition would only get worse in time and eventually become sustained afib and once in afib the ablation procedure has a reduced chances of success. Also, I did not want to take Beta Blockers the rest of my life. After nine months since the procedureI have been without symptoms.
The worst part of this procedure is the time before you go in. I had never been admitted to a hospital and have an anxiety related with anything to do with needles. In my case, I did not experience any pain during the procedure or afterward. Within a week I was exercising again and back to unlimited workouts after 2 weeks.
I think that for endurance athletes one of the best cases of complete recovery is pro cyclist Bobby Julich formally with Team CSC who retired this year at 36. Bobby was diagnosed with RSVT in 1996 and was treated with radio frequency ablations. Bobby went on to place 3rd in the 1998 Tour De France, win a Bronze metal in the Cycling Time Trial in the 2004 Olympics and win the 2005 Paris-Nice Criterium International.
Okay it looks like i am weighing in with everyone else. I absolutely loved smconorm's post...it hit it on the nose for you. With a fib like Jerry also says the incidents increase w. age and that is a fact. I am in my mid 50's and also chose the ablation 3years ago and got my life back. It was a total no brainer and alot easier than a root canal for me. If i had known then what i know now i would have done it sooner and not scared the heck out of my self. I am a huge advocate of ablations and know that they were originally developed for sufferers of a fib among other things. You need to find a one stop doc that does it all...diagnosis, surgical, ablater, EP, all of it so you aren;t running around to all different docs for different things...make it easy on yourself an there are a few rules of engagement here to stay pro active....make sure you choose a doc that has done at least 1500 procedures, ask them how many losses during the procedure, ask them if the know a Dr. Natale or Sheinmann (they are the developers of the procedure and if they don;t buh bye), ask them if they will be going thru your veins or arteries (arteries are now a no no), ask them if they are going to use RF (good), Cryo (good), Heat (not good because of scarring)..i would look at every option you have for sure and whatever you do don't pick up the 9 iron yet i am sure from what you've posted that your love of rowing will not be a thing of your past...ablations are super successful just make sure that the doc tells you that you are in the 70% and above range of success....to me this is an easy decision.......