Welcome to the forums!
I dont really ride all that consistantly...I have other hobbies and I have to spread my time between all of them (kayaking and cycling are my top priorities, as well as mowing the lawn in nice weather, gardening, etc.) But I will give my input!
1: I start with 5 miles per ride in early spring, once the roads are salt free. Right now I put around and just ride for a half hour on my beater schwinn MTB since there is still salt and crap on the sides of the roads. No sense using my good bike. Once the roads are clear, however, I will ride a few 5 mile loops, then work my way to 10 miles then 15 is my normal weekday limit. I work second shift, so I have to ride before work, and anything too much I wont be good for work. Weekends are generally a bike ride with my girlfriend (20+ miles) and a day in the tandem kayak/fishing, or any other of our outdoor adventures (hiking, geocaching, photography...)
2: I think its good to just stay consistant, but work more miles in as your body feels comfortable doing so. 2.5 miles gets easy? try 3.5 miles. then 4.5, etc. Let your body tell you how much you can do, but push it just a little. My first 10 mile ride after the 5 mile loops gets a bit tiring by about 9 miles, but after the first time or two, my body adjusts.
3: Absoluetly! My girlfriend has a Trek 7100wsd I bought her when we first started dating (how could a then 18 year old NOT have a bicycle???) and we ride ~25 miles on weekends on the Canal path, which is super fine crushed gravel/dust. Ive done 30 miles on the same trail with my dad on mountain bikes, so a comfort bike shall be fine. If its more comfortable to you, your going to ride it farther and longer!
And yes, not all cyclists are frail little road racers, some of us actually have some "meat on our bones" so to speak
I will see fairly big people out on the canal path and it actually makes me smile, because at least they are out exercising, no matter how fast they go, it beats sitting home watching tv and eating! Well OK I still enjoy eating, but gotta balance it with exercise!
Speaking of biking, time to take a few miles on my "beater" bike... Have a good day and keep at it, you will be riding for an hour or more a day before you know it