When I have time I like to get my bike and canoe out and do a day trip. Doing a mile or two is not hard but a good day trip is between 8 and 14 miles. When I started at the beginning of the year 6 miles was all I could do before leg muscle fatigue (and butt soreness) did me in. Now at the end of the season I can just make 14 miles in 3 hours with a bit of walking the last few miles. I did a 22 mile bike only trip last week with similar results near the end of it. So I am guessing 1 bike/canoe mile =~ 1.5 or 2 bike only miles.
I usually bike/walk straight through as going down the river is much slower (2 or 3 mph) and getting to the planned boat ramp before dark is critical. The trails are mostly rail to trail and unpaved. Friction, wind resistance and towing an extra 100+ pounds up a gentle slope, are the major impediments. I find a casual pedal of about 5 or 6 miles an hour to be the most effective.
My scheduled exercise time is an hour, 3 mornings per week. Which include free weights, elliptical and recumbent stationary bike (wife's choice for our home gym). I found several related questions they all pretty much say to ride about 2/3 of your hard ride a few times a week. I am looking for a solution that will allow me to do a really hard 20 minute exercise that equate to a 2 or 3 hour casual pedal.
I wish to get fit, therefore should I have a heavy bike?
Does spin class prepare you for a long bike ride?
How long does it take to be ready for a multi day ride as a newbie?
Training involved to ride 175 miles (281 km) in a day
Training Advice for Road Cycling
And because I am over 50- Age-specific information for cyclists