OP, first of all congratulations on your efforts at achieving fitness.
Your bike's seatpost has a quick release. You can adjust the height up and down till you fell comfortable while riding your bike.
You may want to look into a quick tune-up for your bike. Things like cleaning and lubing the chain and adjusting/greasing the hubs can make riding the bike easier and more fun.
Find a group of riders and don't worry about slowing them down. Most local clubs have riders of all abilities.
Have fun, don't overdo it. Riding should be fun, not a chore.
Be safe!