I only did a 16 before my first marathon- OK, so it wasn't pretty on the day, but I finished with a run= walk strateg y from early on. Suddenly jumpoing up to a 20 might just discourage you, and be counter productive.
You won't be as fast as you hoped, but I susupect you'll be OK, as long as you take it slow.
Deasy- as Lit says, it is difficult to know what is too fast or slow for any person, but you are definately looking good for that half mara in May. You could probably manage it now, if you felt OK after 10 miles, so the main thing now is to keep at it whilst avoiding injury.
You will probably need to slow down more once your long runs get over about 15 miles- not necessary until marathon training kicks in- what marathon are you aiming for?
For your back- try some core strenghthening excercises, whilst you are off running due to your foot. As far as the foot thing goes- xrays don't always show stress fractures- sometimes you need an MRI, otherwise wait a few weeks and get a repeat xray. Meanwhile- who is looking at your injuries? If it's your GP, check that he has some interest/ experience in running injuries- if not, the chances are he'll just tell you to stop running, and won't get to the bottom of the cause of the problem. You probably need to see a specialist sport physio who can try to identify why you have these recurrent problems, so they don't start up again every time you tryto excercise.
Hate to disagree with you, Millsy, but Lucozade sport is actually too sweet/ concentrated for ideal absorbtion, and as such might either make you throw up, or drag fluid into the gut, so it can cause problems, unless you dilute it down. I mix it 50:50 in races- they often give you water in a bottle , then lucozade in a pouch a few 100 m's later, so I drink some of the water, then chck enough away to give me space for the lucozade in the bottle, or mix them in my own drinks bottle.
Percy piglets are fantastic! Well worth a try- I think the type os sugar is differnt to gels, so might not cause the same problem.
Check that there's no caffeine in th egels too- caffeine seems to be a laxative!
I know that in the acute phase, the pain can be horrible, but the vast majority of these settle within a few weeks. I am a surgeon- and by the time people get to my clinic, most of them have got better, and don't need surgery.
Keep moving, -even if it's just walking, as soon as things begin to settle, and hopefully you will get back to it sooner than you think.