Many people who start a workout regimen after a long absence are surprised to find how much visible progress is made in a few short weeks, but they often get dispirited when the noticeable changes to their body taper off once they are solidly into their routine.
The key to keeping up with a routine is staying motivated and being positive, and the best way to do that is by setting goals for yourself. It's absolutely crucial that you make these goals tough enough that you challenge yourself to keep progressing, but you also must remember to make them attainable.
If you haven't run more than 100 yards in the past year, don't expect yourself to run a 5-minute mile by the weekend. Likewise, don't get discouraged that you're not a 30-inch waist after one week when you were wearing 38-inch-waisted pants at the start. Goals, as always, should be optimistic and achievable, but they should be pursued incrementally towards an ultimate, long-term mark as your motivation for the future.