Paris-Brest-Paris, a 1200km randonnee that is conducted every four years by the parent club of randonneuring in France. It attracts around 4,500 participants (2003 entry), and is extremely well run, is extremely well supported by the French public, and is an experience not to forget irrespective of whether you enjoyed the riding or not (I didn't in 2003 and that's why I am going back next year to rectify the situation). It is reputed to have inspired the introduction of the Modern Olympic Games, and the creation of the Tour de France. It is open only to amateur cyclists -- professional licence-holders are not permitted to participate.
Fonk, yes, it is entirely possible to do PBP. Much depends on your emotional outlook and determination, irrespective of your triple bypass. Presuming you have a clearance from your surgeon, that is.
The two keys to doing PBP are financial (getting there, staying, enjoying and getting back, plus doing the qualifiers), and how dedicated you are to putting in a moderate training program that primarily comprises longish weekend rides (160km/100 miles should be regarded as the threshhold), mixed with some intensity training during the week. Tolerance of hills would be helpful, even though the maximum height of the route is under 500 metres, there are plenty of rollers to keep you occupied. Oh yes, and rest, which is an essential part of any training plan.
As to those qualifiers, they have to be completed in North America in the year of PBP (ie, 2007), and if you don't have experience with 600s, then you might need to find a back-up for your local 600 elsewhere in case you don't finish that one.
The qualifiers and training will allow you to identify things like bike fit problems (numb butt, hands, feet) and fix them, determine if you are a liquid or solid-fuels rider, how you cope with sleep deprivation, how you cope with night riding, and the lighting issues, and load (what you take with you).
The 600 is seen as the most difficult prelude to the 1200s because it requires a relatively high degree of organisation around a sleep period, whereas the 200, 300 and 400 theoretically can be done without sleep. The step-up to the 1200 is more difficult than it sounds (it's not simply 2 x 600 or 3 x 400 or whatever).
Contact John Lee Ellis of Rocky Mountain Bicycle Club. He ran the Last Chance 1200 last year from Colorado out into Kansas and bck. He lives in Boulder. He's also on the board, I think, of the Ultramarathon Cycling Association. The RMCC website is here:
http://www.rmccrides.com/. The UMCA is here:
http://www.ultracycling.com/
So, yes, PBP is entirely feasible for you.