Welcome Truckman.
The problem you'll run into is that the 42" deck has 2 blades, and the single cyl. 18hp won't have enough butt to spin 3 properly, which a larger deck will have. So, a bigger deck won't be the best thing, without a bigger motor. You'll just turn it into an early terminal, grass beater. Plus, owner will probably not be happy.
Options:
1- Cut yard in sections daily
2- Hire a lawn service
3- Personal investment of equipment that you can take with you, or can sell at minimal loss after your lease is up. Mower racing is an up and coming sport, so you can practice against the wife/son/neighbor kid
As a first time rider user, and the yard description, a Cub Cadet zero turn S series would be a perfect candidate to learn on, and cut your ride time way down. Decent dealers usually will drop you off a trial model to see if it will fit your needs and terrain.