Exercise after Aortic Aneurysm Repair

[hoagie40]

CC replaced my ascending aorta with a graft and repaired (not replaced) my bicuspid aortic valve on July 24, 2008 due to a 5.3 cm aneurysm. I was told the tissue that the graft was connected to looked healthy. I was physically active prior to surgery with BP of approx 105/75, resting heart rate in 50's, non-smoker, and not overweight and I have just turned 50. I have walked every day since surgery and have just started riding my bicycle over hilly terrain this last week. I am taking 50mg of Metoprolol and a 81mg aspirin daily.
My question is that my heart rate during the ride will peak at about 150 during hill climbing, etc but will jump about 10 to 20 bpm after I crest a hill and relax a little. The rate will then start to come down depending on the terrain and quickly will go to the low 90s at stop signs. I have never rode with a heart monitor before and this jump after climbs are over concerns me. Is this normal for this kind of activity? I have gone for only 20 mile rides so far and not sure if I should start working my way back up now or wait a few more months. My goal is to get back to riding centuries again if it is wise to push myself that hard.