In my humble experience as a semi-professional cyclist in another lifetime I would suggest ( as commented by others ) to not go for a wide saddle.
A wide saddle will feel more comfortable for short and occasional bike trips, but when you spend a lot of time in the saddle a slimmer saddle will prove its worth.
The main reasons why people complain when using a slim saddle is that they're just not used to it. But when you start using it during short rides and progressively increase the time in the saddle it won't be a problem.
Also a good pair of pants will prove their worth when riding the bike. Don't go for the cheapest pants, a decent pants (maybe with air pockets) will help your butt from feeling sour.
Additionally you can use pants grease like the professional cyclists do, but that's only required when you spends hours after hours in the saddle.
The knobby tires will not improve your stamina, it will improve your power and that's not the goal when starting to bike. When training to build-up stamina choose a small gear where you must pedal a lot without having to push hard on the peddles. This will allow you to improve much faster in building stamina. The ride distance isn't even that important, the number of rides is.
So my suggestions are:
- Make short rides
- Make many short rides
- Don't go to heavy gears, make it a custom to pedal fast. (a bike computer that measures the pedal speed/cadence could assist ). Heavy gears are only meant to go faster if you are strong enough to push them with the same pedal speed.
- Make many short rides
Tip: don't pedal too fast, 100 strokes per minute a great average pedal speed to train. If you go Chris Froome style it will be too intense.