Since you will follow the easy trekking route that begins and ends at relatively low altitude, there are not many physical requirements for Poon hill trekking. 
Anybody with moderate physical fitness can apply for this trekking. There is no age restriction and young ones and senior people can equally enjoy this trek. So, Poon hill trek is the complete family trekking. It is a holiday hike. Walk leisurely while enjoying each moment with incredible Himalayan views of Dhaulagiri, Annapurna. Fish Tail, Manaslu and other snow- clad mountains and move in and out of culturally rich multi- ethnic villages.
This is the trek that you would want to take if you are the first-time trekker. Take the steady pace and you will reach Poon hill well before you had expected. Having said this, it does not harm to be physically fit.  The more fit you are, the more you will have stamina and the more will you have fun.
While engaging in Nepal trek preparation, you need to pay your attention to the aerobic, physical strength and mental elements. Although aerobic conditioning is not much significant in Poon hill trek due to not being in a very high altitude. There is no risk of High Altitude Sickness in Poon hill area. Better aerobic means better mobilization of available oxygen. Taking aerobic classes before arrival in Nepal helps you in your trek. You can also perform normal exercise like walking, cycling and hiking on not so smooth terrain. It is wise to go for trek in off the beaten trail before joining this trek. Focus on strengthening leg muscles and increasing your stamina level.
Succesful and enjoyable trekking depends more on stamina, confidence and continuity than in speed. It would be a plus point if you could take Tai Chi class before coming for trek as it will teach you to maintain perfect body balance while climbing to the top and descending from the hill. 
Mental preparation is equally important. Although Poonhill trek is a relatively safe activity but you should always know that chances of getting sick or not getting proper rest and sleep and other annoying and stressful things exist in any outdoor activity. Take this trekking as an opportunity to learn and collect experience.