If you 've already tackled a 5k -- .
Even if you have n't -- doubling down on the 10k distance ( that 's 6.2 miles ) strikes the sweet spot between challenging and totally doable .
If you 've already tackled a 5k -- or even if you have n't -- doubling down on the 10k distance ( that 's 6.2 miles ) strikes the sweet spot between challenging and totally doable .