When I restarted cycling after a 30 year gap, about 18 months ago,my average speed was just over 12mph. It is now just over 15mph and I don't go out every day as I have said many times before, I am a fair weather cyclist. So my conclusion is that it is not neccessary to train every day to increase your mph. Once you build up you leg muscles to a reasonable level, riding once or twice a week over fair distance, say 20 miles each time, is sufficient to keep you fit. Of course, training for competitions is different all together.