Although 1600 miles in four weeks represents only about 60 miles per day on average, the challenge of a long ride is to repeat the exercise on successive days. A rule of thumb is that if you can manage 150% of the distance on any single day, you should be fine. Last year I made several trips of over 100 miles in a single day, so I was reasonably confident. However, to be sure, back in March I tried trips of between 50 and 60 miles on five successive days, without ill effect. The only trouble with training rides at that time of year was the temperature. It was almost impossible to keep my feet warm! This despite Seal Skins excellent waterproof thermal socks.
At the time of writing, with about 3 weeks to go, I've done about 650 miles of training rides. There is more to come, including the Isle of Wight Randonnee on April 30th. There are two rides, one of about 35 miles and one of about 63 miles. I'll be doing the longer ride and maybe adding a side trip or two, weather and body permitting.