Your Eurostar ticket is valid for one trip on any local railroad (i.e. not "subway") train to Centraal, which is five to seven times an hour during the business day, and plenty of them at other times. You have to change platforms at Midi, but there's an elevator at the end of the Eurostar platform, if you can't handle the stairs/escalator. If you have to return to London, the reverse trip has a similar privilege. DO NOT overlook Eurostar warnings to allow 90 minutes for check-in Immigration and Customs, in the DEPARTURE station each way.
Note that flying to London to visit the Continent is not always ideal:
https://community.ricksteves.com/travel-forum/general-europe/landing-in-london-then-eurostar-to-paris