The differences in people is huge many times. I remember I was at about 35k of the 50k heading into the last loop and watched the lead runner heading to the finish line and it looked like he was barely working. I was dragging ass and still had 15k left to go.
The farm has done me well at keeping strength but my endurance is not the best. I've kept my hiking legs with paragliding but really need to get my ass running again.
A good option for training is finding some 5k races near you to enter. Small goals lead to bigger goals and it's not about winning but the training and experience of it.
Yeah, some peoples' level of fitness is just off the charts. I've started supplementing with 3mg of creatine daily to help with endurance. From what I've read it really helps. I need to work on the endurance as well. I've done fewer long bike rides but do climb up/down seven flights of stairs for 20min three times a day along with kettlebell exercises and burpees.
Good idea about registering for a 5k. Those will be few and far between in MN until spring but would be a good motivator for me.
I bet there are more races than you think. We have runs every month through winter with our favorite the Frostbite 5k in January. Last year it got canceled due to sub zero temps and the course workers had to be kept safe but that is a rarity, we've run it in the teens F temps prior years.