Well, one simple solution - enter another race? Seriously, find one about 4 to 6 weeks away, so you know you have to train for it, and can't just sit back down - and go for it. You have a target now, and believe me, the second race is a LOT easier than the first! You know what it feels like to be in a group, to race that far etc etc.... as long as you don't pick something much much harder you should hit a PB next time...and then the motivation continues further...
More advice - set yourself a weight target, set yourself a pace target, set youself a distance target. At the moment you need measurable performance, if you see improvements you'll keep going. There's no point saying "I'm going to lose 10kg" - say i'm going to lose 1kg...and do it...then another 1....little boosts every week or two are much better than aiming for a big boost 2 months away and never getting there. Conversely, if you don't hit a target, don't beat yourself up, and don't quit just because you missed one. We all miss targets... it's not the end of the world - just make yourself more determined not to miss the next one....
3 to 4 times a week is perfect - if you run more than that whilst still being new and overweight then you're asking for injury troubles... Just to give you an idea - I run 4 times a week...and at the end of August I completed the 60mile long Ultra Tour Peak District... so you don't NEED to be doing more than that at the beginning - just enjoy the runs, and make them count 