I'm going to bring up a related question to consider of how much time you have to mow?
Your actual mowing speed may depend on the thickness and height of the vegetation you are going to mow along with how slowly you need to travel in order not to turn over on hilly terrain, especially if it is uneven, rutted, has woodchuck holes, etc hidden in the vegetation.
If I understand what you've posted, you have about 7 total acres you want to keep mowed. Most fields are rough enough that you may only be able to mow at about 4 mph. With a 5' cutter, you would be able to mow an esimated 2.18 acres per hour so it would take you about 3.2 hours to mow all 7 acres.
Mowing Calcuator | How many acres can I mow in an hour