on even a good 125cc 90kmph is the top speed that it (and hence the rider) is most comfortable with. If you can try to vary your route a bit so you can spend as much time as possible out of heavy traffic where you have to be constantly on the alert. Mapping a route out on a GPS prior to the trip may help you with this. I only just discovered my phone's ability to be my guiding voice last night. I'm dreading the bill though.
Given that you're new to this scootering caper it could well take you a while to build up the stamina. If you can ride as much as you can for as long as you can on weekends to help build up the stamina.
On my Vespa GTS250ie I do a total of about 27km either way to & from
work a day. Given that my Vespa's top speed is about 140kmph I can very
comfortably sit on the freeway altnough often take the long way around
as I spend far less time in heavy traffic and the route involves far
more enjoyable twisty bits & scenery, things which add up to a more relaxing ride.
Today 50 (2,500km), Montego. Fly (2,500km), Le Grande (7,000km), Vespa PX (200km), Vespa GTS250ie (46,000km), now looking at the world through Ruby coloured glasses on a Vespa GTV300ie Vie Della Moda.