I spent the summer getting into cycling as a way of building fitness. So not just a couple hours a week to supplement my running, but the amount of time required to actually move the needle. That meant 14-20 hours per week, and that was in addition to squeezing in 50-70mpw of running. And if there's one major complaint about that process was the amount of time spent on the bike compared to what I felt was a tangible improvement to my fitness.
And now that I'm back to just running 10-12 times a week, so plenty of doubles, I feel like I have all the time in the world. Not to mention the efficiency of that time spent running versus cycling. Oh, and I have a full time job, wife, two kids, and enough time spent outside of all those things, including running, to make me happy. But cycling? Man, that's a time commitment, certainly makes running doubles seem easy.