Firstly, do not buy a bike from Walmart, Tescos, etc. Next you will need to spend about £350 minimum.
The reason for this is simple. The better engineered the machine, the more reliable the components, the lighter the overall weight. A full susser for £80 will have hideous gemetry, weigh a ton and fall apart, or at least out of true, at the first bump.
What sort of riding do you want to do? Road, general cycling to and from work, XC, All mountain, etc. This will dictate that style of bike that you buy.
If this is about fitness, then you are looking at a road or hybrid bike. Mountain biking is harder on your joints and takes more grunt. You cadence tends to be better than road users. However, you have just had an op on your foot. This might not be the best choice. Stick with Road or Hybrid.
Cycling is a component game. Stick with brand like Shimano (Dura Ace), SRAM. Get a frame with a good quality aluminium. 6061 or 7005. Also, buy from your local bike shop. Not a car place that also does bikes, or a supermarket. Do not buy off of the internet unless you know how to set a bike up.
I would recommend Giant, Trek, Specialized, Dawes or Claude Butler.
However, there may be another issue. If you are flat footed, like me, running and especially cycling can cause problems. I.E. It can throw your knees out. Go and see your GP and preferably talk to your physio and find out what stretching exercises you might need to do, or if there will be any other problems.
Let us know how you get on, we might be able to offer you advice on your choice of cycle, if you post links to models, or give us a budget.
Luck