Plus it's a learnt problem. We're the last generation that can remember a pre internet pre mobile phone era, where such a thing as no signal wouldn't be an issue, because there wasn't a signal to be concerned about in the first place.
Experts are idiots though. When I started (from a background of mtb like the op)I did a long run of 11 miles in my first week, 13 the week after, and 17 the week after that. Could of carried on if it wasn't for tearing my knee cartilage on the third run