Ah all my sympathies to you!! I had exactly the same thing happen last year. Had an emergency cerclage at 24 weeks, and full bed rest thereafter till delivery. The little rotter didn't emerge till 41 weeks after all that (turns out I have the mother of all cervixes ;)) so it was a loooong wait!
Have to say, if someone had told me when I had the stitch that I was looking at another 15 or so weeks of bed rest I would have gone mad. But as it was, we were fully expecting him to come early, so actually we just lived week by week, hoping and praying for the best. Every week that went by just feel like such a bonus, and that was all we focused on till around 30 weeks. If we could get to 30 weeks, though it would have been traumatic and a challenge, I had been assured (by the very wonderful consultant looking after me) that it would be manageable. I had steroids at 30 weeks as the stitch slipped, the cervix shortened even further and they were fairly sure he was on his way, so prepared ourselves for that but never eventuated. So another 10 or so weeks ensued.
On the sanity front- I watched a LOT of box sets, did some freelance work, did some research for friends on new ventures, and spent WAY too much time researching pushchairs ;).
On the fitness front, I wish I had been more fit going into the pregnancy but such is life. Nothing much to be done for that. I walked the length of the hospital every week or so when I went for my fetal fibronectin and exam and that was pretty much it. But there were some websites I used from around 35 weeks on positioning/birthing exercises. I'll link it. Have to say, bed rest is rubbish on helping you prepare physically, obvs. My skeleton, and particularly my back, was really not in the best condition for giving birth having been sat on my arse for four months. So after 37 weeks, and with a huge baby, I literally did everything I could to shake it out...acupuncture helped, did loads of walking, Swiss ball, yada yada.
You won't go mad...you think you will, but you don't, weirdly. Partly due to the first weeks being totally focused on keeping the baby in. The rest you get through with good friends and cupcakes. It's amazing how much time you can waste when you put your mind to it;).
spinningbabies.com
Good luck!! If I think of anything else I'll add. 