I have been trying to figure this out, and I think there are three things about it that make it much suckier.
1. I start off downhill and end off uphill. At home it is the opposite, so I end on a high note, so to speak, rather than feeling rather more slow and useless.
2. The dog is on leash. This is annoying in some ways, though he is very good and runs right along beside me, but I also loose the distraction factor by watching him run around and generally be gleeful.
3. Long straight flat trails/sidewalks simply suck. Little kids, if the trail winds and is narrow and interesting can walk remarkable distances. Put them on a long straight road and they're begging to be picked up within a hundred meters. I think I am the same. They are dreadful, plus you don't have the distraction of watching the footing nor the delight in rounding the corners and seeing the new vistas change.
Persistence and determination is more the order of the day, and that just t'ain't so much fun.