As others have said, focus on increasing the time. Don't worry about the miles, don't worry about the speed, either. When I decided to start mountain biking again, 10 years ago, having been off the bike for 20 year, I started pedaling on a stationary bike I had in my house. 20 minutes was hard, but I did that, and worked my way up to 30 minutes and then increased my effort at 30 minutes until I felt confident I could hang with a beginner ride locally. Riding outside, the focus has always been keep progressing to ride the same loop(s) faster while continuing to increase the riding time with longer rides/weeks. Time in the saddle is more important than the speed, though.

Maybe try backing down on the speed and focus on getting 2 laps, that should take about an hour, and get comfortable doing that much. Or go 1/4 of the way around on your second lap and then turn around, will give you 1.5 laps.

Yes, sometimes I have miles to go, and it's hot as hell or the wind's blowing in my face, and my legs are fatigued and I'd rather be done. But I'm not done, I have to finish...then I'm done. So, when you get to that point, back off the effort a little and set small goals for yourself...look forward to getting to the next gate. Stop and walk for a minute, sometimes that helps, something I've had to do at the end of a long ride and then I get into deep sand...dying. My Friday loop is 44 miles these days. I ride it home, and I get 22 miles from home with no shortcut back, so I don't have the choice to just call it a day. I'm 55, and down to just under 200#, and was up to 220# a couple years ago.