In TrainerRoad I'd say the best plan to start with, if you follow one of theirs, is sweet spot base. Older riders may have to throw in additional recovery weeks and perhaps avoid the high volume plan.
Things I track:
FTP, weight, FTP/kg, maximum power at various durations, heart rate response (training has made my HR a lot slower to rise over time during sessions), total training stress - TSS (using TrainingPeaks as I can include outdoor rides and rowing on there) and the associated metrics CTL (fitness) ATL (fatigue) and TSB (form).
TSB allows me to gauge how much to train in the days leading up to an event (balancing recovery and training when tapering) and also to track recovery. To give some context, an easy hour of endurance has a TSS of around 40 and the Brevet Cymru will have a TSS of 750-800. Whilst with a TSS of 40 you're easily recovered by the next day a TSS of 450+ takes several days of recovery. TSB is negative during training and at the end of the taper for an A event it should be positive but not too high as you don't want to lose too much fitness.