First, see a good doctor and get checked out. At your age and with your history, this is an absolute necessity.

Then, assuming you are healthy, start walking every day, every season, in all weather except a blizzard. When you can walk the two miles up and back without being winded, increase the distance. When you get to five miles a day, then start the bike again—at one mile every day, then increase the distance.

Why such a slow approach? Because your body does not have the physical memory of being even moderately active, and building that at your age takes time. I'm 68, and I have been very active all my life. I have also had to come back from multiple major medical problems, so I know from experience that this takes time and has to be done gradually.

But first see a good doctor!