These are really only accurate in quantifying the number of steps taken ( walking or running).The accuracy for distance depends on your calibration of the length of stride which has a large margin of error no matter how you tweek it.As someone who has run nearly 100,000 miles in the last 30 years, the best calibration is simply take yourself down to the nearest running track ( or gym threadmill) and see how long it takes to run a mile at 3 paces, leisurely, medium and fast as possible. Then a pencil and paper is all you need to compute how far you have travelled! Anyway, experienced runners dont bother about distance - use TIME as the measure of performance and if you want to measure degree of fitness and/or energy expenditure , buy a heart rate monitor instead - which is an exact science!
Anyway enjoy it, and dont bother about pace
cheers