Lost my MOJO

I've been in training for a little while now. I was quite heavy at Xmas 06, just over 17st, so my intial training concentrated on some weight loss, and to try to bring my general fitness levels up to a reasonable baseline to start some dedicated distance training. I lost 2 stone and was quite happy with the way i was progressing. With the NY marathon as my ultimate goal in November, i had a decent amount of time to train for it.
About 3 weeks ago, i came back from 2 weeks holiday in Egypt. Prior to the holiday, i was quite happy with my training and had managed to get in a 13 miler, which wasn't very pretty, but at least i got round. I did some light training on the treadmill whilst on holiday, which i thought would keep me ticking over, but since i came back, i seem to have completely 'lost my mojo'. Its been quite painful really, and i dont seem to be able to run like i did before i went away. I'm sure it will get better, but with 3 weeks to go, it is a little bit worrying. I am tending to get tired very quickly and my stamina seems to have reduced. I ran round Richmond park last Sunday, and whereas before i would have got round the 7 miles without too much pain, i was suffering towards the end and had to take a couple of rest breaks to walk and get myself together.
I had a half decent session during the week with the GSK running club, which has picked me up a bit so i'll see how i go this weekend.