A friend has been riding his bike to work for the exercise and for the money.

A friend has been riding his bike to work for the exercise and for the money. He is fairly heavy, but he is still in decent shape. He has an older low-end mountain bike that has needed some work.
He has been making a classic mistake: using gears that are too high for the speed. This causes two problems:
It wears out bike parts too fast
It puts his body in the wrong mode for good exercise.
The bike:
When you push hard in higher gears (the little gears in back), you place much more stress on the chain and on the cogs (the gears on the freewheel). To make it worse, he sticks with the same gear all the time. Many people make this mistake. This makes the one gear wear out quickly and it always takes the chain with it. You know it’s too late when the chain starts skipping when you accelerate. It’s better to use larger gears (in back) for your bike and for your body (explained later).
Bigger gears use more of the chain. More chain links are pushing on more teeth for less wear. Also, lower gears move the bike easier so there is less force applied to the chain. It’s just mechanically better.
My friend thought the front chain rings were causing the skipping, and he was trying to remove the right crank arm. He didn’t realize that it requires a special tool that screws into the crank are and pulls it off of the spindle like a gear puller. He even tried to knock it off with a hammer. This does not work, and he ruined the crank arm.
I explained the real problem and warned him that he will be buying a new chain and a new free wheel. However, he can make it last a little while if he avoids using his favorite gear (this is a better idea anyway, as you will see later).
I gave him a new crank arm and loaned him my puller. I told him to put the new one on tight, and he got it done. He was surprised at how much better the bike rode. His old crank arm was steel, and the new one is alloy. Alloy crank arms are better at transferring the energy into the chain instead of flexing and wasting it. You can certainly tell the difference. Actually, upgrading your crank set is probably the best and most noticeable upgrade you can do for the money.
I also gave him a new left crank arm so the set would match, and he would notice even more difference. I warned him that it’s more important to get the left crank arm tight so it won’t fall off. The left crank are take much more stress than the right because it is turning the spindle and then the right crank arm to pull the chain; The right crank arm only pulls on the chain and doesn’t put stress on the spindle. He told me that he was having trouble with the left crank arm falling off before. He tried several things including Loctite to hold it on, but it kept falling off. The fact is, once it falls off once, it’s ruined. There is nothing you can do to repair it. You have to get a new one. Again, he noticed the improvement in ride quality.
The body:
When you use high gears and pedal slow, you are not only putting too much stress on your bike, you are working in anaerobic mode. It’s the same as lifting heavy weights and doing fewer reps instead of lifting lighter weights and doing more reps. You get tired much faster, and you will not have as much stamina. I supposed your power muscles could get stronger, but there are better ways to do that than riding a bike. Cycling is an aerobic exercise like running is. Use lower gears (bigger gears in back), and pedal faster.
Try to find a comfortable speed that lets you pedal fast. You might feel more winded at first, but it should go away. That is your lungs opening up, and they might not be used to it. You will be able to ride a lot longer, you will go faster, and you will get it shape much easier.