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Your mention of riding on sidewalks indicates your newness to bicycling. I recommend checking the laws in you area regarding sidewalk cycling, as it may be illegal, for one thing, and if legal, you need to see how sidewalk cycling affects your right-of-way status around other types of vehicles and pedestrians.
I also saw mention of trails. If you by any chance mean designated pathways meant for multiple types of users, keep in mind that attaining any level of fitness may be impractical due to the other types of users crowding those facilities. If you mean actual off-road bike trails, then that is good.
I am not familiar with the bike you bought, so this may not apply, but a knobby-tried, true "mountain" bike, is not the best machine for commuting or other on-pavement use. See the Commuting forum right here on Bikeforums.net for tips on that aspect.