What is the best way to build a strong base before the summer time?

Hey there, this is my first post on this forum so hopefully I don’t make any mistakes here! I’m starting to take cycling pretty seriously and I’ve got an indoor smart trainer that I’ve been using for the past month or so. I am wanting to take my training to the next level now but I’m in a bit of an odd situation in that I’m only able to ride every other week. My time during my week of riding is pretty flexible though so I can put up to 15 hours in if need be. I live in Canada and there’s snow on the ground here until around may so I’m wondering what the most effective way to build a solid base come summer would be? What would be some good training ideas for the week that I can ride, and also some recovery/workout ideas for the week I can’t?

Here’s some side info if it helps: FTP is 210 or 3.5 w/kg @60kg