I've been planning on losing weight for the Ironman in September so it would be a little bit easier. I know I could finish at 250lbs or so but why make it such a challenge. Losing weight over the winter didn't happen. Some triathletes also seem to think they can eat whatever they want since they train so much. I'd say I half thought that. Most people who found out I weighed 250 never believed me. Even when I was 275 playing football in HS no one ever believed I was 275.
To the point...My starting weight was 250 and now I'm down to 236. I've been quite surprised at how much faster I've been able to run at the same effort. It's probably a combination of eating better and weight loss. Supposedly:
Every 1 pound lost = 3sec/mile faster
So, if you're running 10 minute miles and you lose 5 lbs, theoretically you should be able to run 9:45 miles at the same effort. Granted there are countless variables but bottom line, if you lose weight you should be able to run faster at the same effort.
Recently I've been able to run training runs at at least 45 seconds - 1 minute faster with the same heart rate. I say training runs because I haven't raced in over a month and I never run as fast in training as I do in a race. The biggest indicator was a 12 mile run I did 2 weeks ago. Normally my long runs would be around an 11min/mile, this run was 10:15/mile at about the same effort.
I would tend to agree with the 3 seconds per mile theory but it's probably on average. I think the more you weigh the greater the increase would be at first. Then when you start getting to a "normal" weight, you might only see slight increases per pound. Then I would assume your pace would increase if you started to become underweight. I'd like to get down to 220 for the IM. I chose 220 so I can still remain a clydesdale for a little longer. However, like I've said before, if I'm "too fast" (IMO) for the clydesdale category I'll switch to AG.
In short, lose weight, get faster. Simple enough.
As an aside to this post, given my weight, I once tried to seek out other people with similar paces who weigh 250 pounds. I was trying to figure out if muscle mass (massive legs) played a role in not being able to finish 16-20 mile runs without walking. I'm like Gimli from LOTR, very dangerous over short distances. I even wrote to Runner's World but didn't get a response. I think Ben from Ben Does Life is probably one of the few. However, I think he is a little different since his muscle mass is probably lower and hasn't lifted weights most of his life. This is why I hate indoor triathlons on spin bikes. I can't spin my legs at 130 rpm for too long.
I got a response from a former body builder but he wasn't quite as big. He noted that losing muscle mass made him much faster. Another guy on a forum even said "not many people your size do what you do. You're, pretty fast when considering your weight."