Undiscovered Scotland is wonderful and I use it for all my planning for Scotland.
Enjoy Lincoln but be warned, the Cathedral and Castle are at the top of a very Steep Hill! If you are staying at the bottom of the town, there is a shuttle bus that runs from Silver Street that takes you up to the Cathedral and Castle.
Although the Castle grounds are free to enter, it is well worth buying a ticket. Ask for a castle day ticket which includes the grounds tour. It is the same price as the 'normal' day ticket. Not only does this give access to the Victorian prison (fascinating) and Magna Carta, it also gives access to the wall walk, with its views over Lincoln. It also includes a free tour of the grounds which covers the history of the castle and Lincoln as well as explaining what you can see.
The Cathedral ticket price also includes a free ground floor tour. Not only will this point out where the Lincoln imp is, it also highlights so many other thinbgs that are easily missed. Again it is an excellent introduction to an incredible building. Depending on what day you are there, they also have other tours. The roof and tower tours do invove a lot of steps but again give a completely different perspective. I loved the roof tour. When buying your ticket ask about turning it into annual pass (no extra charge) which means you can go back into the cathedral next day if you want.
There is also the Old Bishop's Palace which is due to reopen (Friday, Saturday and Sundays) from the beginning of June after major work.
Both the castle and cathedral have good cafes so it is easy to spend nearly a full day at the top of the town.
The Tourist Information Centre is between the Castle and Cathedral and has maps of Lincoln as well s knowledgeable staff.