Even the most challenging of the long distance routes have long sections of easy track and flat road - and the inclines on most of the routes, the Cornish Way and the Edinburgh to Aberdeen routes are examples, will lead you from sea to low moorland and back to sea level again in a short space of time, so the variety and scenery changes easily and quickly. The 200 miles Coast & Castles route in southern Scotland and northern England easily combines moors and coastline with only one challenging section. 

Take as long as you want over a route. Forty miles is the best if you are holidaying the cycle ride. Some people will do 70 miles a day, others 20 miles.  Some cyclists complete the C2C, one of the hardest routes, in under 15 hours. So take two days or 10 days. The success of the C2C route in the north of England has shown that people of all ages and fitness can enjoy cycling at their own pace. As long as a sensible approach is made such as carrying the right clothing (and that doesn't mean expensive cycling gear) then the routes are there to be cycled by everyone at whatever speed you want.

And, if you're really tired you can get off your bike and walk for a while.