Deep Water Running is a zero impact training alternative to regular running after it was recommended by Coach Joe Beer.
Done correctly it works the cardio respiratory system and many of the same muscles as regular running, without the impact. Obviously, it doesn't work the calf muscle as there's nothing to push-off, but the research suggests it's way better than nothing. (Open to read more)
* Firstly, the belt. I use the Speedo Hydro Belt
which at £21 seems more compact and less RoboCop than others I've seen in the pool being used during Aquacise classes.* If you're visiting a new pool, tell the lifeguard what you're doing. Seriously, you don't want them thinking you're struggling to swim.
* Don't try to use it when others are lane swimming, you're too much of a hazard. Done correctly, your arms and knees will be pumping and you could easily punch a swimmer in the face. Wait until it's 'free-swim' time, get into the deep end, then work alongside the wall.
* Go in with a fixed time and session in mind. The longest I managed was 70 minutes and that was only possible because I had a structure and something to listen to.
* How to do it - Google Deep Water Running for videos. You can even buy sessions to download. My advice is to get the arms and legs working together as if you are running. Keep knees high, but don't let your legs perform a cycling motion. The advice is to lean slightly forward, but I found this promoted the bicycle action, so I limited the forward lean.
I could travel from from the deep end to the start of the shallow part of the pool in two minutes, so let's call each of these half-lengths a 'length'. Every length down the pool, facing the wall with the clock, I'd do a sprint - 20" on the first length, then recover while completing the length and returning to the start, 30" on the second length and 40" on the third. There's a lot of recovery time in that set, and initially it seems too much, but do this for seventy minutes and you'll get quite a work out.
* But there is no getting away from it - it's boring. I listened to Podcasts to pass the time, and rather than buy one of the many Waterproof mp3 players
I found an easier way. 
I played the podcasts from my iPhone in a waterproof case
by the poolside (have a towel handy in case you need to handle the iPhone) and listened to it on the same pair of Bluetooth headphones I use for normal running. These SoundPEATS
cost just £14, stay in my ears, and sound really good - I believe a test by The Gadget Show found them almost as good as the Beats pair and they were literally a tenth of the price.I'm delighted to say my ankle has improved thanks to the biomechanics work by my podiatrist. The belt is back in the gear cupboard - long may it stay there - but thanks to it and deep water running I don't feel my run is starting from scratch. I haven't improved since last October, I'm just picking up where I left off.