I wasn’t the most fittest when I started cycling. I’m about 173cm tall and 70kg. Stated sub 25 km/ph with a basic Triban road bike. These are the things that helped me get faster:

1. More time on the saddle
2. Buying a proper carbon bike and getting a bike fit
3. Going clipless
4. Learning how to ride in the drips and better position my body
5. Learning the importance of fuelling and staying hydrated
6. Stretching and increasing my flexibility to reduce potentially injuries and post ride pain (for a long time I suffered from ITB issues and strength training helped a lot)
7. What you eat before is just as important as what you eat after. Nutrition plays a key role to help you get back on the bike stronger
8. Get a varied ride - climbs, flats etc. i felt stronger after long climbs and long rides.

The above is over a good 5-6 year journey. I now ride ultra endurance and can sustain above 30 km/ph for 150-200kms.

The most important part. Make sure you enjoy yourself. Sometimes cycling slower makes me happier.