I'm kind of old (39), kind of out of shape (5 km in 36:30, but hey, it's hilly) and determined regardless to run a half-marathon at the end of June. I've never run more than 10–12 km before, but my brother-in-law and sister are marathoners, know me, and are confident I can manage it.
I love my erg, though. I've just started this program
this week to try to get myself up to snuff for the half-marathon, and I'm trying to do upper-body weight training on my "rest days" (nothing too extreme: about 15 minutes of bench presses, situps, curls, and alternating shoulder presses/pullups. More keep-muscle-while-losing-weight stuff than any attempt to get huge or anything).
I was wondering if swapping in erg work on some of the "recovery run" days would really impede my progress. It's still pretty shitty weather here, and not heading out in -8°C weather for a half-hour low-stakes run when I could happily hit the erg for 30 minutes would be an asset.