To tackle very long events the single best thing you can have done for preparation is to have accumulated a huge volume of training at low to moderate intensity spiced with focused strength sessions and a small amount of event specific intensity over many months.
By event specific intensity (for mountaineering and running) I mean weighted steep uphills for climbing/mountaineering events and long uphill running intervals for running events.
It is hard to reconcile your “feeling fresh and strong” with “legs feel heavy even when rested” comments. The first sounds like you are ready to perform at an optimal level. The second statement sounds to me like your legs have not recovered from a deep fatigue caused by a heavy long term training cycle. If you’ve been doing a lot of ME this will leave your legs heavy and dead feeling and depending on the length of the ME block it can take 2-4 weeks for your legs to feel fresh again. The delayed training effect of this ME block will then last about as long as the concentrated training block was. This heavy legs feeling is VERY unlikely to come from not enough stimulus to any muscle fiber type, especially the FT. What you are feeling is fatigue.