This is harder than it used to be, given that Avalon Hill is now out of business. However, several larger mail-order game stores and online stores still have backstock. Remember to get the softbound book and not any RuneQuest "deluxe" or "standard" boxed sets. You should also get something called Gods of Glorantha, since it has a great variety of spells in it that appear nowhere else. If you're planning on running your own world, you don't really need anything else, since most other RuneQuest products are expansions on a world called Glorantha. You might consider getting something called Vikings, which presents a great deal of detail on the norse culture.
There are a lot of things you don't need to convert. First, your maps, non-numeric descriptions, and history need no changing. Your world is still your world.
The most important difference between RuneQuest and AD&D is that RuneQuest takes a pagan and medieval model for their magic, and AD&D has a 20th-century industrial view. In pagan and medieval European society, magic was not the exclusive property of only a few powerful individuals. Everybody had some small charms they knew. While RuneQuest does presume this, you are not required to go along with it, although it does make for a more authentically pre-industrial world.
If you decide not to change the AD&D distribution of magic, simply do not assign converted characters any magic unless they already know some. If you do decide to permit this, you will need to decide whether the common folk practice what RuneQuest calls spirit magic or a low form of sorcery. If they practice spirit magic (and spirit magic is a very folksy-feeling type of magic), then permit non-magical characters (any class that does not have spells) to have one point of spirit magic for every two AD&D levels (round up). If the peasants practice "low sorcery", then give all non-magical characters a score in the Intensity sorcerous manipulation equal to 5% times their level and let them know one sorcerous spell per every two levels (round up) at an equal skill. In any case, do not exceed the INT limitations of RuneQuest. For classes like the Ranger and Paladin, who do not have spells at first level, treat them as "non-magical" unless the character is of high enough level to already have spells.
If you have decided to use the AD&D industrial 20th-century approach to magic, then you must cut references to gaining magical spells from character generation for new characters.