Starting over...how to increase mileage&speed most efficiently
After a loooong Spring and an even loooooooooooonger Summer where on-bike time and mileage reached near-record lows, I find myself basically starting over fitness wise. I've also changed riding formats, moving from a LWB touring setup to a SWB go-fast, high racer recumbent. The change in riding position is a result of seeking a better ride for my back - so far, that's working out well.
Up to three weeks ago, I was only riding the LWB on a 10-12mph, 20 miler once a week at best throughout the summer. Things have opened up time- and commitment-wise and I can ride daily again. Unfortunately, my first three weeks back have been all but miserable. Like I don't remember it being quite this bad, slow, painful any time previously.
Part of this is definitely due to adjusting to the new bike, literally getting used to it. But most of it is I seem to have lost my fitness/endurance. I've definitely lost my spin.
A couple local guys are suggesting that I am currently putting too much emphasis on increasing mileage without paying enough attention to intervals (specifically they're saying swap out two of my 4 - 11-13mph, 20-30mile midweek rides for 2 - 5-10 mile, higher intensity rides) [I take Wednesdays and sometimes Sundays off while I'm at slow steady metric distances on Saturdays].
So I'm looking for ideas, experience and/or real-world comments about what I can do to increase my endurance and speed from granny-walks-faster-than-me-slow to being able to ride a single stop 17-20mph solo century (something I approached regularly a couple years ago but never quite reached).
My long-term goal is to complete a Series next year. A couple years ago, I was riding monthly 200k's in the 10hour range. Different bike though and nothing longer than 150miles.
Schedule-wise, I'm hoping to complete a 300k in Feb/Mar and the 400/600s between Apr and Jun - before the Sun decides to hate me(us all) again.
Open for comments...