Hello Cam,
There are so many avenues that you can take for fitness gains. Be patient and stay consistent with your training as nothing will happen overnight.
If you haven’t already, I think that one can really benefit from setting some specific goals (short and long term) which should both, be reasonable and attainable. You asked if it is better to do shorter rides or longer rides and also mentioned wanting to gain “some weight”.
As far as what is better training for you, ask yourself where it is that you want to go with your cycling. Do you want to race, cycle centuries, brevets (distance series), perhaps branch out and do a triathlon (run, bike, and swim), or just general fitness …. And so on. A long term goal can be to pick your first event, if that is something you wish to do.
Also take the time to assess how much time you can or are willing to commit to cycling, training, weightlifting, or any other activity each week. Having a good grasp on your goals and your schedule can help you decide how to approach your training regimen for maximum effectiveness. Do you live in an area that experiences the different seasons such as winter? If so, figure winter training into your training plans as that will be a great time to build strength and dial in your nutrition.
Speaking of nutrition … be more specific than wanting to gain “some weight”. Is this weight to improve your strength, appearance, or both? Do you want to ride and look good (Lance Armstrong) or just ride good, who cares if you look emaciated (Tyler Hamilton), etc. Set a reasonable goal – I want to gain “x” amount of pounds by Christmastime … and of course, you can always adjust and readjust goals along the way.
Have you considered getting a trainer or a coach that can work with you on the total package? Perhaps join a local club or team through a bike shop or other network ~ make new friends, training partners, find quality resources (coaches, sports nutritionist) to help you jumpstart your new fitness program.
And finally, welcome back to cycling … make sure that you train within any limitations you might have if there are any lingering issues from your injury.
Just remember that what works for one person, does not work for the next. Talk to alot of people, do alot of research, and really start to understand your body and how it reacts to your training, your eating, and your sleep and stress levels.