When I was in the Army (a million years ago), they would make us run in sand carrying 70 lbs. of equipment. There were two ways to increase "capacity". The first was to tell yourself that you would make it over just one more hill and then you would dump the equipment and tell them "no more". However, after you made it over the hill, you would tell yourself, "Just one more hill". This short term focus was helpful.
The other thing that helped expand capacity was shouting encouragement to the person next to you. This seemed to boost your own capacity.
I don't know if this is applicable in your situation, but I found it helpful. If you experience difficulty riding on the bus and start shouting at the person next to you, it might not work the same.