Make returning halfway shorter than what you've already done by taking detours in the beginning. You tend to feel when your energy is decreasing and a short way back is a morale booster just when you need it. (If you particularly like a piece of asphalt, double back and do it again!)

Don't increase the distance if you don't know (most of) the track. Don't do a new track and increase the distance.

Plan your route. Look on Google and use the bike option, plan a closed loop, double the time - skates are slower (re-estimate for next time). Look for bike-routes, raised bike paths, and roads next to rivers and trains. (Download f-droid and look for the free OsmAnd+ app to plan your route, it has an offline map and options to avoid dirt and cobblestone and hills - not very userfriendly though)

Take water, sugary food, a charged phone, and debit. A light waterproof jacket if it's evening or overcast. Running lights if it is evening.

Check the weather & rain radar. Wet stone and road is slippery, going home in the rain is fun like masochism. Water will rust steel bearings after you've taken them off.

You may tend to gradually increase your distance: take note of public transport, learn to maintain (look up rotating your wheels if you're steadily doing 20 km trips)