I haven't ridden much gravel but my guess is that you really don't want to pedal until you spin out on gravel. Is 40 a little scary? If that's the issue and this is in hilly terrain, maybe you're doing it right? If not, then simply focus on shifting and turning the cranks. On pavement, even on descents, I like to see at least 50w on the meter, indicates that I'm at least making some effort at a good finishing time. And as in post 2, a trainer or rollers are nice for getting used to continuously turning the cranks. On many roller rides, I set my resistance so that I'm just below VT1 - where breathing rate increases - and just sit there and pedal at that resistance for an hour or two.
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