If bike hasn't ben oiled for a while the gear changeR could have seized completely. Best get someone with some bike knowledge to look at it.
if its stuck on the big ring then it could be either that the derailleur is seized, or the cable is seized, or both
It felt like 4mph today. 8 miles each way, but I use 1/4 cycling miles as running equivalent.
I think you can be way more generous than that.