Well, why not try and get up earlier to get in more miles? I, like others on here sometimes commute to work, but other times I'll ride
before work and taking my son to school. I've gotten up as early as 5:00am and sometimes 6:00am to get in either 20-35 miles. It's definitely rough
to get up that early, but if that's when you have the time then that's when you do it. Defining decent is different to all on here, so I guess
you could intensify your ride to make 13 miles more of a workout. I also agree when you say 13 miles is better than 0 miles.