Proper bike shorts and shoes with clipless pedals and a 25 lb. used road bike will make you a heck of a lot more comfortable, as well as faster.

Many years ago when I was starting to ride seriously, I made a mental prioritized list of the things which had helped me:

At least a $150 used road bike that fits and has clipless pedals.
Proper clothing: helmet, shorts, jersey, bike shoes, arm warmers, leg warmers, wind jacket and vest, long and short finger bike gloves.
Saddle bag with spare tubes, patch kit, multi-tool, pump.
Heart rate monitor.
Cateye Astrale bike computer with cadence.
Learning what to eat and drink.
Good tires.

Back when I was strong, the group of fast riders I led would frequently average 18 during the summer for 60-80 miles and ~4000', riding all out on light road bikes, and having trained for years to be able to do this. To average 18, one has to be able to hold ~23 on the flat unless one lives in Florida. No, no hybrids. A Cat 2 came out with us sometimes on his fixie, but cut it short after about 30 miles when his legs started to burn out, which is why he bothered to come out with us. That's the reality of average speed. It ain't easy.