The Alshich points out that the second posuk of the sedra is in the wrong place. If the Torah is going to told us how old Yitzchok was when he married it should have done it when it reports the marriage. Further why does it say Lo L'Isha and not L'Isha.
Because the story continues with the birth of Rivka's son Easu one might well have wondered where such a child came from. Look at the brother of the Kallah as an oracle of the child who will be born from the marriage. As she came from a father, a brother and a place of evil men one would conclude she is the source of the evil of easu.
It is therefore important here to establish the motives for Yitzchok's marriage to Rivka and that he did not marry the wrong person or "below" his station. Usually people marry below them for one of two reasons. Either the girl was so beautiful ( P.G. Woodehouse yarns were full of sons of Dukes marrying stunning actresses) or there was no alternative partners.
The Torah emphasises the fact that Yitzckok was forty years old when he married. Forty says the Talmud is the time in a man's life when he achieves wisdom. The choice was made through wisdom and not through youthful infatuation. The sending of Eliezer hundreds of miles to collect Rivka was because she was the perfect match for Yitzchok. They were in fact chosen for each other forty days before they were even formed in their mother's womb as the Talmud explains. That's why we have the extra words "Lo L'Isha" to him for a wife, she was already Lo L'Isha.

I remember as a boy of nine watching a television play. It concerned a business man who was under a lot of stress and decided to go to a "retreat" for a weekend. He arrives at a monastery which is run by monks who observe a vow of silence. He is reminded of this condition of his stay at the Monastery and shown inside to his room. After unpacking he goes down to the refectory where he sits on a rather spartan bench to eat his evening meal. There are other people who have come to get away from everyday life. The door opens and in walks the most beautiful girl he has ever seen. She sits opposite him and he smiles at her and is met with a demure smile in return. It is love at first sight. The play shows them walking hand in hand in the rose garden and going down to the cellars to watch the monks making Mead. At the same table where they first met he passes her a note. It is Sunday night and tomorrow the weekend's guests must leave. The note speak the words that the promise of silence has forbidden him to speak. "Can I wait for you tomorrow and take you home in my car?" Again the vision of loveliness nods demurely. The next morning sees the young man hurriedly packing and rushing downstairs to collect his expensive sports car which he drives to the main entrance of the monastery. He gets out and sits against the car anxiously looking at his watch. Eventually the door opens and his perfect beauty walk toward him. He walks to meet her and for the first time since they met they are able to speak. He stops, smiles and says "Hi" she looks up at him and in the broadest cockney accent says " Oi Hullo dux" his face resembles a balloon which has had it's knot untied. There is no thing as love at first sight. There is infatuation and attraction at first sight but not love. You love a person, a whole person not just the pretty face. It needs wisdom to realise this and to reqalise that you have to look for the right husband or wife, in the right way