There's really no need for a structured plan unless you're racing seriously or intend doing some TT's.
Plenty of miles in your legs first off. We're coming up to winter, so join a club and get some long miles in before the New Year ( am I really saying that!) Try to sprint with the faster boys to the cafe stop and stay on their back wheel on the climbs!
Two sessions I would try after Christmas, once a week for each, indoors or out :
To ride faster for longer -
2x 20 minute intervals. Warm up, 20 mins at a faster speed than normal, recover for a bit, another 20 mins harder riding, then warm down spin to your back door.
Sprinting -
Warm up,
20x 15 second sprints on/off ( 15 second sprint, then 15 second recovery and go again),
Recovery spin 5-10 mins,
10x 30 secs on/off sprints,
Recovery spin,
5x 1 min on/off sprints,
Recovery spin as long as you need before falling off bike.
Best done on a turbo IMO and it's not compulsory to do all three to start with. Doesn't look much on paper, but I find this extremely hard - but I am old and decrepit!
At a later date you can think about heart rates, training zones, power meters etc. if you're still keen.
Alternatively, just ride 200-300 hundred miles a week at a decent pace. That'll do the trick!