I'm a bit closer in age and speed so maybe I can help.
These are just my rules of thumb as a relatively fit tourer who tours with his wife, but has a background in triathlon and endurance mountain bike racing.
1) Regarding Speed: It's probably safe to anticipate an average of 11-14 mph. Seriously. It sounds slow, and I usually average 18-20mph on road rides, but that weight makes a big difference.
2) Concerning Planning: My safe estimate is to plan for 55miles/day and every 7th day off. Give or take based on weather, gradient, and your morale.
3) Regarding April: My wife and I are planning a Northern Tier-ish kick off to a lengthy multi-year tour and we have been planning an April 1st start in Seattle. We know this is borderline "Insane" but plan to make allowances by detouring around the North Cascades (Hwy 20 is typically closed until mid May) and by budgeting plenty of days off the bike in a motel. Starting in Banff in April is not recommended. Not for a first time trip.
That said, you'd be surprised how warm you could actually be with proper tights, outer rain gear, long sleeve jersey, warm socks and shoe covers, headband, and winter cycling gloves. Crazier things have been done and it's not that what you're proposing isn't possible, but rather that it could be very less than enjoyable. Just some food for thought.