Why is your PHONE telling you how fast you can ride? From what you say about 12 miles being 'nothing', I'd bet you're faster than THAT.

I ride MTB exclusively -- commuting, errands, fun, family rides, whatever -- and I can do 12 miles faster than 1:12 ANY TIME.

If 12 miles is nothing, then 24 a day should be little more; BUT, you may just feel a bit tired for a few days, that's all.

Sounds like you're looking for 'permission'; hell, JUST DO IT.