Yojimbo_ said:does my goal seem achievable?
Yes
Quoted post said:is my protocol reasonable, or should I consider something different?
Sounds good, but don't feel you've got to press all those efforts right up to the max for the duration. It's more than o.k. to back off a bit so that each effort is reasonable and so that you don't come to dread training sessions. For instance a lot of folks strive to set records for 2x20 sessions, it's fine to back those off to 90% of FTP, they'll still require focus but they shouldn't kill you. One approach is to work down in intensity but up in duration during your training blocks. So maybe start your 3 day block with 2x20s at 90-95% of your FTP, follow it with a couple of 30 minute efforts at 85-90% and finish the 3 day block with a straight 45 minutes to an hour at 80-85%.
Quoted post said:should I try to mix in a longer L2 or L3 ride once a week to maintain endurance until I can get outside again? If so, should I use one of my "off" days or convert one of my "on" days?
I don't plan any indoor L2 sessions but often drop into L3 (see above) and those are definitely considered training days. I end up getting L2 work as rests between harder efforts and during warmup and cooldown but don't have L2 on my schedule as a target workout.
Quoted post said:when should I switch to pure L4, or should I?
I don't know that I ever get to pure L4 work. L4 is hard enough that I can't do it for more than an hour to hour and a half per session even when broken into intervals and I sure can't do it day after day. I might add more L4 sessions later in the winter but it comes at the expense of building CTL so I never go to a pure L4 work week.
Quoted post said:when should I mix in some L5, L6, and L7?Thanks in advance.
I add L5 work in late winter about 8 weeks before racing starts and then only one day a week. L6 work comes later and if you're just focusing on TTs you really don't need much if any L6 work. I'll start L7 work as soon as I get outdoors and would do it all winter if it wasn't so miserable on the trainer. Again you shouldn't need much if any pure L7 work for TTs but might do some for general riding quickness, fun and the ability to acelerate on demand. You can always do some microinterval work like Bill Black's Hour of Power workout for the latter.
Your goal seems very reasonable given your starting point. Just make sure any plan you make is something you can carry out till spring and don't hesitate to back off when you need to. A post on another thread this evening reminds me that there's risk of folks taking this stuff too literally. If you burn out on any plan and stop training then that plan has failed. Consistency for the long term comes first followed by consistency in the weekly schedule followed by time and intensity of each workout. IOW, sticking with a plan is the most important thing and sometimes that means easing up a bit or taking more time for other stuff. SST work is great that way. You can get really good training benefit across a range of intensities and don't need to hammer the high end all the time to see improvement.
Good luck,
Dave