New Challenge Type - Percentage Improvement

I would like to see a challenge type where people can compete for best percentage improvement of their selected distance (1km, 5km, 10km, 1/2 marathon or marathon) with a set duration for the challenge (eg 2 months). Therefore runners of all levels can compete in a challenge, regardless of the distance they want to run or the pace they normally run at - it is all about personal improvement based upon your own personal standard, and the best way to do this is in percentages (a bit like how they work out which team wins each week in the biggest loser when they compare weight losses in percentages as the starting total weight of each team was different).