Good luck Steph, hope you get to 10M. I've been bad this week and not run at all since the parkrun last Saturday. Then yesterday I kicked the bottom of a chair and bruised right in the middle of the top of my right foot...
HM training plan starts tomorrow (rest day!) and due to family coming today won't be out now today... I really have to step up again. Also going to start the diet and shift the remaining stone or so if I can over the next 2-3 months.
Biggun, I'd probably keep on and make sure you're used to running 25 miles a week before your last 4 months marathon training - but why not now do a 2 month 5K training, then a 2 months 10K and 3 months HM training with a race entered at each endpoint (keep your long run up though)? It'll keep you interested and you'll be building up rather than stagnating. You can probably easily knock over 5 minutes off your 5K time with some decent training, which will pay off later as you up the distance and lose the weight. My view is slow running is necessary for your long run of the week to build endurance, but (and this reflects my experience only) doing some speedwork, increasing your lactate threshold/VO2max gives you a faster base for your long distance stuff - plus it is much more interesting than just plodding every time you head out. Realistically I'd never get the miles in if I always ran at my long slow speed, I just wouldn't have the time and I'd get a bit jaded I think as I like to push my PBs every so often. I know Andi trains differently and it works for him but I thought I'd throw in my 2p.
Forerunner 620 does it all (220 does most and is more up to date than the 310XT), but considering your list is down to 2 very old models, I guess you want something fairly reasonably priced - 310xt seems about £170ish. I had the Suunto T6 and the garmins with integrated GPS are way better in my opinion
I'm one of those who hates smartphones while running - I find them awkward to carry (even an arm band is restrictive) and you can't glance at it easily unlike a watch (plus where I run there is no signal so the phone it fairly useless in itself). But I guess they have their uses if you just want to log and dissect after a run, and are free if you already own it!