I feel a bit embarrassed about replying as if I'm some kind of expert, but can really identify with DD, Jenny, Joanne...
I started running in May last year when I got fed up with the gym and wanted to be outside more. My 'base' level of fitness allowed me to jog for 20 minutes without stopping the first time, which I was so..oo proud of. I got some decent shoes and within 2 or 3 weeks was able to run for 30 minutes. The things I've read suggest this shoule be our first target a novice runners - 30 minutes without stopping, regardless of distance. I 'did my own thing' in terms of run/walk, although I'm sure these schedules work well. Some people advocate a walk/run strategy even wehn you are experienced and very good! The really important thing, especially if you are doing just running rather than run/walk, is to go slowly, even if you feel you could hammer on a bit faster - think jogging not sprinting. This way you spend more time out there on your feet and are much less likey to exhaust yourself or even injure yourself.
About a month into my running I bought a polar heart rate monitor and had the same experience as Jenny. I found it very discouraging to be advised to run so slowly, when I actually felt fine in what I was doing - it felt counter-intuitive. But then (on the advice of someone posting on the forum) I got hold of John Parker's 'Heart Rate Monitoring for the Compleat Idiot' and it all fell into place. He uses a differnet formula to work out max heart rate and percentages - I think the usual ones are far too low for me (my Max rate observed so far, in a 3 mile race is 12 beats higher than the standard formulae predict). Anyway, get this book if you can, because the hrm really begins to make sense. It has helped me move on to 'speed work' and, I believe, kept me well and really enjoying running.
Re. weight - I'm still about 2 stone above weher I 'should' be, but don't think this should be seen as an obstacle unless you are setting out to WIN races - lots of large people can and do enjoy running.
And finally, races... My initial goal was RFL in July 2003, but I found myself entering a local 5 mile in June. The feeling of completing that race was indescribable - it's doen wonders for my confidence, especially as this is somethign new I've taken up since turning 40. I'm usually in the last 5 in the races I have entered, but so what? I still really enjoy it and feel very supported by fellow runners.
Anyway, enough rambling. DD, I also like to find out everything when I start a new venture. But running is something where you really learn from experience - keep a diary and watch yourself change!