I've been working my way through a modified version of the "Couch to 5K program", the major modification being that I only run once a week, so I'm progressing 3 times more slowly than you're supposed to. I've actually been quite pleased that I've still been able to make quite a bit of progress despite only running once a week. It seems like this is a lesson I keep having to learn: just because it might be "optimal" to train 3 times a week or whatever doesn't mean that you fail to make progress if you do less than that, especially if you're consistent.
However, the one drawback to running once a week is that if I have a crappy run for whatever reason, I kind of feel like I've blown it for the week. Two weeks ago, I had one of those beautiful runs that are probably the reason why I keep running even though it so frequently sucks - I felt good, I held a good pace, finished strong, felt great afterwards.
Then last week, I hit the pavement ready to do it all again, and like two minutes into my run, my left calf cramped up. Walked it out, stretched a bit, tried again. Nope, instant calf cramp. Briefly considered just running anyway, and then did the grown up thing, went for a walk, and came home and spent some quality time with the foam roller and the lacrosse ball working the knots out of my muscles.
This morning, I went out to run, and while I didn't have any cramping, it wasn't what I would call a great run. My legs were pretty tired, and while I "ran" the full length of time I was supposed to, by the last few minutes I felt like my pace was really lagging. It made me wonder if I was cheating myself out of cardiovascular benefits by running when my legs were tired and couldn't keep me going fast enough to keep my heart rate as high as possible.
The Fitbit Charge HR actually makes it possible to semi-intelligently answer this question. I compared my heart rate data from the awesome run of two weeks ago and today's crappy run.
The awesome run:
18 minutes in peak heart rate zone
14 minutes in the cardio zone
14 minutes in the "fat burn" zone
138 average bpm
The crappy run:
17 minutes in peak heart rate zone
13 minutes in the cardio zone
18 minutes in the "fat burn" zone
133 average bpm
So, there's a difference there, and it might even matter if I were training for competitive endurance running. But from a general fitness point of view, I can now confidently say, "Yay, crappy runs are awesome!"