if you went to your doctor, he'd likely be following the most recent US guidelines.
Here they are, JNC-7, which you can read yourself:
http://www.nhlbi.nih.gov/guidelines/hypertension/express.pdf
"Individuals with a systolic blood pressure of 120–139 mmHg or a
diastolic blood pressure of 80–89 mmHg should be considered as
prehypertensive and require health-promoting lifestyle modifications
to prevent CVD."
European guidelines are similar. No drugs.
That's if you don't have other risk factors.
What lifestyle mods? You are already obviously exercising :) How about the DASH diet (reducing sodium while adding potassium, etc)?
The guidelines also recommend home monitoring. You can check your own monitor against other readings that you get at a regular checkup or elsewhere. Every home automatic monitor I've ever had was accurate. You can get your own stethoscope etc if you want.
Btw, there is a very strong trend to disregard diastolic pressure, especially in anyone over 50 years old.
I would address your BP with your doctor now. Although not real high, it's still high. Your body will be under a lot of stress while running and you want to make sure your BP doesn't rise too high. Our blood pressure can change throughout the day. My husband and I have used a home monitor for years with much accuracy. In fact years ago on a whim I took my blood pressure while at the drugstore on one of the machines and it was high. I called my doctor and he also said that these aren't accurate, but wanted to see me. It was accurate, and that's the day I started on BP medications. My husband has a lot of health issues and has to check his BP regularlyat home, and it's always within a few points of what the doctor gets. I don't think it's anything to be real concerned about, and for you to be 65 and in this good of health is awesome!! But I would see your doctor and make sure all is well with your BP so that you can run the race in a good state of mind, worry free. Even if you have to go on meds, you'll still be able to do the run, just without worry! Your pulse is excellent. Good luck in your run, my son is an ultrathoner.
I assume this is the systolic figure you are giving, but the diastolic is just as important.
I see nothing wrong with those numbers, 140 is not seen as particularly bad. It depends
on how fit you was before you started training, it can take years to get a relatively unfit
body into good shape. I also assume you are using a home BP monitor, which are not
particularly accurate and should only be used for a rough guide. In rehab these monitors
used to report my BP as 160/90. Every week I would visit my GP to have it checked
properly, with a stethascope/cuff and it was actually 130/65, a huge difference and this
was straight after a half kilometer walk.