Generally the cheaper the ticket type the more restrictions, both to hours of travel and route, and ability to break the journey - which is what you have asked about. Breaking the journey is the phrase you want to ask about when buying your tickets. Many off-peak return tickets allow one (or none) breaks on the outward journey, and unlimited breaks of journey on the return.
What you mustn't do is diverge from a straight line, nor backtrack, nor go back and forth or retrace your steps nor visit anywhere twice.
Some tickets give you up to 5 days for the outward journey and up to 30 for the return, others require all travel in both directions be on the date printed on the ticket.
Anytime and Off-Peak tickets don't change (except in price, going up twice a year, regular as clockwork) and can be purchased whenever you want, from well in advance up to moments before the train pulls into the station.
Advance - with the capital A mentioned by Marco - tickets are capacity controlled (like airplane tickets) must be purchased well in advance for the best prices and will either be more expensive or not available at all closer to travel. Advance tickets are valid only on the exact train (and on some train companies the exact seat) printed on the ticket.
Your question is just a tad vague so more precise answers aren't possible. I don't understand when you say:-
I have seen some off peak ones but they aren't yet available for when I am going to be traveling.
Can you elaborate please? When will you be traveling? Where did you look for your answer? What parameters did you put in?