What type of bike should I get?
First - a lot has changed in 19 years, but not much. Some bikes are lighter, faster (and a lot more expensive) than 20 years ago. The difference between your Hardrock and a new one today is small enough not to matter for most people most of the time. 

I am with @Blam this - if just wanting to get fit its arguable a heavy slow bike is better - its certainly cheaper than upgrading.  

If comparing against yourself is the goal, any bike will do. 

If comparing against others, one thing is more satisfying than passing some random cashed up middle aged exec wannabe on a $10000 super bike on your 20yo Hardrock - seeing their face as you do it.

If racing against others and winning is your goal, you need a new bike. If keeping up with a group is a problem to the extend you do not enjoy it, a new bike would be a wise investment. 

Valid reasons for a new bike is if the Hardrock is a poor fit and/or you are uncomfortable riding it. If the Hardrock is worn out (many 19yo bikes have less miles than many 19day old bikes, so age is not a good guide) it may be better to replace it than spend money fixing it. 

Another valid reason is if, having put down hard earned money - against your husbands wishes, it makes you feel bad about not using it and motivates you to keep riding where you would have stopped riding had you not upgraded (i.e. it gives you the will power to keep riding). 

One good reason not to replace it just yet - wait till you have a few more miles under your belt, and have established a pattern of use. At the moment you are getting into it. You may find you like single trail MTB riding, or get into 100km road rides. In a few months you will know how you are using your bike not how you plan to use it. 

By not changing the bike, you don't have to ask 100 questions (What kind of bike, what brand, how much to spend.....) that have no correct answer.