Hi Kate, welcome to the forum 🙂 Well done on committing to improving your health and fitness! What insulin regime are you on? What are your levewls increasing to? Exercise can be tricky, especially when it is something more than you have been used to, so it might take a little while and some experimenting to try and find out what works well for you. It can be quite an individual thing, as is often the case with diabetes - some people drop like a stone the moment they step on a treadmill, whereaas others (like you and me!) find that levels are steady, or even rise after. I would say one of the main things to look out for is the timing of the exercise in relation to your insulin doses. I have found that, for me, I need to eat/inject around 1.5-2.5 hours prior to exercise. This allows both the food to digest and the insulin to get working properly. If you don't have enough insulin circulating, your levels will rise as you can't process the glucose properly. After a run my levels might still be increasing, but then things will fall back and I will usually need to reduce my insulin doses later as the exercise makes me more insulin-sensitive.
If you get the timing right and the post-exercise rise is still unacceptable then what may be happening is that, having depleted the glycogen stored in your muscles, your liver may be releasing extra glucose from its stores to replenish them - if this is the case then a small amount of sugar/glucose should stop this (a paradox, you'd expect levels to rise anyway with the food, and they might but not as much). I usually have something like a Belvita biscuit after a run. Try different strategies, test often and you will gradually build up experience of what works best for you 🙂 Something else to consider is that different types of exercise might affect you in different ways - for me, running is fine, but gardening requires constant top-ups of jelly babies! 😱
