Definitely keep your old bike, at least until you get more experience
and identify any equipment shortcomings that are holding you back.
Forgetting what these "hardcore" riders told you, how did your bike
perform? How did you feel? On the trails I ride I regularly see all
manner of dual-suspension long-travel bikes. However, most of the
riding in my area is smooth, fast, undulating singletrack which is
actually a lot more enjoyable on a fully-rigid bike. What are the
trails like in your area?
After riding for awhile you'll figure out what you need. You may find
you need a front suspension, or a full suspension, or no suspension.
You might find you need wider gearing--or you may dispense with the
gears altogether and go singlespeed. Just ride and see what your body
and the terrain demands. This may or may not entail a new bike.
Good luck--mountain biking is a great adjunct to road riding.