that website gives the calculator but then states that it is based on an unrealistic assumption...so basically not worth even looking at
important note: this calculation is based on the (unrealistic) assumption that the change in weight is not accompanied by any other change. In reality, losing weight may take you to below your healthy weight, in which case it will not result in an improvement in performance, and may result in illness.
For me its hard to isolate because generally if I'm losing weight whilst training (usually after an injury when I've put on weight) then it's hard to know how much improvement is down to weighing less and how much is just down to being fitter through training. Too many factors at play to accurately estimate.
Plenty round there like Sale Harriers, Manchester Harriers, Chorlton Runners and Altrincham AC. The first two for definite have track work but still do a lot of other sessions depending on your interests.