The best way to get in shape is to gradually increase your ride distance. Some new riders attempt to ride 20 miles or more from the start and end up being injured. You need to work up to longer distances a little at a time. You will get there, just be patient.
Judging from your comments that you had great difficulties climbing some of the grades, you may want to consider changing the gearing to gain more of a mechanical advantage. I believe your rear cassette has 18 teeth. The specifications for your 7-speed Shimano system indicate that a 22 tooth cassette is available. The Momentum website does not show what type of crankset gearing is standard. You can go as small as a 36 tooth crankset on the front.
I just turned 78 years old in January. Both of my knees are bone on bone.
I purchased my e-bike (Trek Allant + 8s) to help reduce the stress on my knees and allow me to continue cycling into my 80's. To help increase the mechanical advantage I changed my rear cassette from a 11-42 to a 11-46. I also changed the chainring from a 46 tooth to a 40 tooth. This combination works perfectly for my location. I can climb a 10% grade with no issues, using the largest spocket (46 tooth) on the rear and next to the highest motor setting (Sport). I never have had to use the highest motor setting (Turbo). If I pedal with maximum effort in Sport mode I can achieve 25 m.p.h. I typically have a moving average of 13-15 miles per hour.
I would have a conversation with your dealer about the possibility of changing your gearing to help with hill climbing and your driveway. Shimano documents says it is possible. See attached example of the availability of different gearing.
I know little how these re-gearing techniques apply to an internally geared hub system. Your dealer should be able to help you with this.
For your application with an internally geared hub you need to use this type of calculator......https://www.sheldonbrown.com/gear-calc.html
I have re-geared all my bikes over the last 15 years in this manner to reduce the stress on my worn out knees. I also am able to achieve a reasonable moving speed average while having the ability to climb steep grades.
The attached example of a gearing calculator shows some of the calculations you can make. Note: this calculator is for conventionally geared bicycles.
Calculate cycling speeds at a specified cadence given chainring, sprocket, tire and wheel sizes.
www.bikecalc.com
What percent grades do you have to climb?