A new shock will not make you faster on the road. If all you want is some more road speed then I'd recommend getting a set of slick tires. You can buy new tires for probably less than $40 total. That will increase your speed significantly on the road vs the knobby tires that are likely on your bike.

As far as how long will it last? It's tough to tell. You really need to quite frequently check for loose bolts, noisy berrings, broken cables, and any odd noises you may hear while riding. Make sure that you are checking your headset for play pretty often as well. Doing these things will not only help you make the bike last longer, but will help to insure that it's safe. That bike is not intended for 20mi/day riding so you'll have to be careful to keep it in as good of servicable condition as you can. I'd recommend checking out a quality site with repair instructions and spend some time learning about how everything works.

Here is a great site

Bikes are not as simple as people think and in order for them to work properly a lot of different parts need to interact correctly.