Exercising - foolishly?

Every person that knows I'm sick has told me to "stay active". Although it can be annoying advice, I do agree with them because bodies work better when they can move. Last Friday, I went to a pool and I swam laps in the slow lane. I zig-zagged around for about 20 minutes, rather gently, I thought. However, for the last 4 days my right chest muscles have been so unhappy. The problem with vague advice like "stay active" is that I have no clue about the parameters.

Is working out more aggressively and longer better? Should I stay flexible? Should I do martial arts to get out my aggression or perhaps body building? This too is trial and error, just like most things in life. Here's what I know so far:

• Wrestling or sports where you get tossed around like a rag doll aren't ideal if you have weak bones (in my case due to cancer).
• Intense running can be a bit hard too. Running on grass is better than concrete (it's sad that this wasn't obvious to me).
• Cycling is lovely and low impact. Plus you can watch HGTV if you're at a gym.
• Walking is good, especially with a buddy.
• Light free weights can give some aesthetic definition to an illness ravaged body.
• Swimming and yoga are good but go about half as hard as you think you should (it would seem that concept is lost on me). If you do stay flexible and your back goes out you can use your feet to find clothes and dress yourself. I'm so grateful that this happened to me because I was not found both nude and immobilized.
• Congratulate yourself and try not to feel sad if you can't do what you once did or what other, hotter,  people are doing. I tell myself that I'm going to age more attractive than I was. Just watch it's real :).

In times where you're body wasn't it's strongest what exercise did you choose? What has been effective?

Although you guys aren't the most enthusiastic commentators so far, I will wear you down!