Hi. Sorry for the delay in replying. I am able to ride for 4 hours on my own and average 18mph. I should have said that each week now I am doing 2 rides of 60-70 miles. One is a ride where I aim to keep an average of 150bpm for the 4 hours, although it's up and down of course depending on terrain and wind and really to stop me getting bored.
The other is a more gentle group ride. I normally go out on my triathlon bike for this and we potter along at 17mph in the group. My heart rate sits around 120bpm for the 4 hours.
As work permits I have been doing short 13 mile TTs in the summer/autumn but I have let these go now we are getting towards the tail end of the year as work is too busy during the week.
What you are saying sounds like a brill idea, I will start building in speedwork in January. Given that I can already ride for a good long while do you think the base building will still help??
I can recommend following the training plan(s) in Chris Carmichael's book " The Time Crunched Cyclist". I used this to get my IM cycle time under 5 hours (yeah, 4H 59min!). Be warned that you can't use this training plan and continue regular tri training so set aside 8-12 weeks to get your bike up to speed - the fitness will stay with you for the season. The sessions are brutal!