yeah your best bet is to just spin easy loads, but
a long time in the saddle...shoot for ever increasing
cadences but don't kill yourself. even ride slower
rather than faster...but leg spin speed is mandatory....

keep it fast 80-90-100-110-120 rpm
every 5 then every 10 minutes jump to another cadence
and run it for a minute or two or ten...

put the time in the saddle. then the real test will be the big ride
and it will be easy as pie if you do the work now and focus on the above