So, with a possible stress fracture of my foot and an aircast boot, running is out. Cycling is allowed and I'm doing lots of that (over 300k in June - first cycling for about 8 years!), but I've also finally started some seated routines as suggested by UNM (thank you! I didn't know these things existed). I've tried a nice seated abs routine from SparkPeople fitness which definitely worked my core, but wasn't really cardio. I then found several 'hurt foot fitness' videos on you tube from Caroline Jordan. There are lots (she's even wearing a boot in one of them, although I haven't tried it). I've only done workout number 4, and I keep my feet off the floor whenever my legs are moving (which maybe makes it a tiny bit harder, but it's doable). It definitely gets my heart rate up and I end up pretty hot and sweaty too. Thought I'd post a link for anyone who wants to give seated cardio a go - whether injured or not! In my experience, once you get started on something it's OK - but I can waste ages looking for that start point.
So, dither and prevaricate no more! A couple of possible start points if you have to remain seated: youtu.be/SMc9H2TmXUM
or if you prefer something a little calmer, shorter and with less cardio youtu.be/d2-B-BPmlPA