remember that good form, sharpened individual finger dexterity (practice scales and simple melodies), and right-hand strumming and pick holding technique all contribute to good rhythmic strumming. I am a lifelong musician and only took up guitar a couple of years ago by taking an intro class over summer at a community college-I can say that all the technique and practice with chords, single-note melodies, scales, and playing along with an instructional CD, all helped me tremendously.
Lastly, I will add that a properly set up guitar with low action and not a classical will also help a lot. I sat in class with other students who had improperly set up guitars, with wide (classical) necks, and theirs were remarkably more difficult to play and learn from. I have a Martin Dreadnought knock-off bought used, and with a little research found that it was a perfect fit for me. If you’re in the market for a good beginner’s guitar look up EZ Play guitars, which come in both Martin and Sigma (the brand I went with); their playability and the one class I took allowed me to continue playing and have made learning a pleasure.
Remember that if you have good form and a properly matched guitar, playing should not hurt or be painful. Finger fatigue and muscle weakness is natural, but you shouldn’t be feeling strain if all those factors are taken into consideration. Good luck!