Well, that depends on the trek. The longer the trek, the harder it will be to cope with your fitness. We recommend you to choose the trek you are comfortable with. However, we’re not saying you shouldn't go for a long trek. If you’re physically fit and mentally determined, you can definitely nail the longer treks.
Yes, you’ll definitely need a trekking guide if you want to relish the trekking experience in Nepal. In fact, trekking guides are compulsory for trekking in Nepal. You’ll get to acquire a lot of information from the trek if you hire a trekking guide. You’ll get to learn a lot about our country. Also, if you’ve any trouble during your trek, your trekking guide will always be there to assist you.