Good for you! What a great goal and Tahoe should be absolutely beautiful.
I’m not very athletic either and survived the Seattle to Portland bike ride last year. 100 miles is very doable! You’ll do great!
Are you doing any local events? I might suggest the Chilly Hilly at the end of February and the Urban Assault (which is kind of a partner-scavenger-hunt) in mid May.
Good Luck!