As I get older I get slower going up the fells
- more weight less speed. But in general I'm more interested in my surroundings than what speed I'm going at. Age is only relevant when you can't do it anymore - walking I mean
As for walking fitness - we all have to start somewhere.
On average I would say on decent ground where you can stride along 3mph would be a decent pace, 4 mph when you can really stride out. But in reality with stops to admire the views, take photo's, look at rocks, plants etc the actual average drops down to about 2.5mph.
Up slopes is another thing altogether - up the steepest slopes you can expect to average about 1mph where there is a bit of scrambling involved. About 2mph for most other slopes - but that depends how often you get to walk uphill. It takes a few walks to acquire some mountain legs, and it doesn't take long to lose them as well.
Distance wise - In the lakes with a lot of ascent I wouldn't want to be walking much further than 15miles - maybe 8 to 10 miles in the winter. Long distance paths wise - anywhere between 15 and 20 miles a day for most routes. Sometimes it's a bit further to reach accommodation and the nearest watering hole.