Screen shot 2011 01 03 at 16.16.00 RunKeeper downloaded 1 Million times in 4 days after going freeIn a well-timed promotion, fitness tracking app Runkeeper Pro has seen an impressive one million downloads in the past four days.

The app, which allows exercisers to track their fitness and share their workouts on a map, usually costs $9.99 to buy. However, the app is currently available for free throughout January in an offer designed to catch all those New Year’s resolutions to get fit.

It certainly seems to have worked. On its blog today, Runkeeper has announced over one million downloads since the start of the promotion. Yesterday, CEO Jason Jacobs told VentureBeat that RunKeeper Pro was downloaded 171,000 times on New Year’s Day – in itself apparently over 10 times the normal download rate for one day.

Despite missing out on all those $9.99 download fees, Runkeeper should still do well out of the promotion. The word of mouth alone should help brand recognition right through the year, while the startup heavily promotes RunKeeper Elite, a subscription service with additional features such as fitness alerts and real-time sharing of workouts, on its website at a cost of $4.99 per month or $19.99 per year.