Best time to exercise

I am on insulin but I find that exercising in smaller amounts (15-20 minutes) after a meal helps my insulin sensitivity.

If you can fit in a bit after breakfast (not before, I think as that will make your liver try to dump more glucose into your bloodstream - not as such dangerous if your levels are below 10 or so, but can be annoying to watch on a meter), go for a walk around lunch if you have the time, and perhaps do a walk after dinner too? Sounds like a lot, but could probably be fit into 45 minutes total, really.

On weekends when I have more time, I tend to go for a 2-3 hour walk, but that's just out of choice because I love walking.

If you run and go to max of what you can do, you sometimes find your blood glucose having an initial rise, but it will then come down lower than starting point after an hour or two. If you do moderate or easy exercise, it seems it most often will just slowly drop. Both options are valid as long as you know what to expect.