Amir I am a little anal with anything I do the 1:40 did not come easy plus I want to be under 1:30 before the body starts to tell me its to old (i am 41)
I started from a low fitness base-plenty of walking but no other exercise since leaving school 20+yrs ago, followed a run walk plan and went out on average 5 times a week all seemed to be going well (to well) so i started to increase distance and speed to quickly and got 2 injurys close to each other -Planter and ITB'S.
After easing off for a while I was able to restart my training plan.
When I could run a couple of miles without stopping I timed myself for a mile at flat out pace-this was end of feb and i did 7.45 mins giving 95%.
I then stook to my plan gradually increasing distance and putting in some faster shorter runs- 17 weeks after just been able to run a mile at 7.45 pace I was able to run 13.1 miles at this pace-all i did was keep to the plan and didn't miss a session.
I think the difficult thing is keeping injury free,setting goals that to yourself are reasonable and not worrying if other think "1:40 HM thats a slow mid week training run for me"
GOOD LUCK