What causes Heavy legs?
In your case, it sounds like too much volume, too soon. It sounds like you need to develop/adapt your body & its muscles more slowly to the stress placed upon them.
I immediately rested, probably too much.
To prevent the feeling of heavy legs, you must increase your mileage only at a pace your body is comfortable with, become more flexible to relieve stress from your muscles and joints and promote recovery.
"After a long ride", then walking upstairs, would make even the accomplished cyclist's legs ache.
You just need to tough it out & hang in there, good diet, good sleep, continue with the cycling, I think some lowerbody weight training wouldn't hurt & hopefully your body will adapt.
I have Raynauds. It affects my hands & feet (extremities) especially in colder conditions but its never affected my legs.