Trust the plan

So I've been following my trusty marathon training plan and my new bodybuilding program. So far it's been going well. I'm into week 5 for the bodybuilding program and week 6 or something for the marathon training. The issue I'm facing now is that the marathon training hasn't really started, as in I'm just in the build up stage so I feel like as long as I get in 40-45km a week I'm doing fine.
I'm trying to be smart about it, but that's led to stupid mistakes, haha. Take this week for instance; Monday I'm feeling tired (I did run 28km over the course of the weekend) and sluggish so instead of enjoying the sunny (albeit windy) day I did an indoor HIIT workout, telling myself Tuesday would be a longer run day so I should conserve my legs. WRONG. I did my usual lunchtime back and biceps weight training and after work decided instead of running the indoor track I would run the local trail. (Honestly, it wasn't the track per say, but the fact the skating group has decided to take over one lane of the track to do land training for 30 minutes, thus forcing a cluster effect on the only other track lane-for half of the track length. I get a bit rage-y when people aren't actually running or walking on the track I've paid to run on.) So anyhoo, I get to the trail but it's still really snow-filled and icy. I plug through, determined to get in at least 6km even though I managed to forget my spikes (they would have helped a lot!). I ended up taking to the country road at one point to finish off a total of 8kms. Not exactly a longer run in my books, I didn't even hit double digits. So yeah, I "saved" my legs for nothing. Ugh.
Today, my usual cross training day is taunting me. Should I jump on the dreadmill when I get home to grab a few kms or stick to my routine of cross training? Deviating from the plan this week doesn't seem to be working well for me, so perhaps I will stick with my cross training. There's always my run commute home tomorrow and Friday!