The easiest way to know how fast or slow you should be moving for fitness is to get a heart rate monitor. Find your heart rate zones and work within those zones. The basic formula is 220 minus age, then multiply 65% to 75% to get zone one, 80% to 85% for zone two, and 86% to 90% for zone three. Some of you will be slightly above these percentages and some of you will be below, depending on how conditioned your heart is.
Talking and breathing are good indicators of where your zones are as well.
Zone one: evident by ease of breathing and talking while working.
Zone two: you are still talking while having to take a second to catch your breath.
Zone three: you need to take a breath after one or two words.