There are 21 missions extending over a distance of about 650 miles, or roughly thirty miles apart. There is a wide range however as the main criteria was that each mission be about a day's journey (by horse / wagon) and the terrain varied a lot.
Roads / Transportation
Books that I have read include: El Camino Real. The Golden Road: The Story of California's Spanish Mission Trail by Felix Riesengerg, Jr.(1962). Two other books are Helen White's El Camino Real de California (1956) and Edwin Corle's 1949 book The Royal Highway (El Camino Real). Charles Outland published a book that might be helpful: Stagecoaching on Camino Real Los Angeles to San Francisco 1861 - 1901.
I checked the definitive books that Fr. Zephyrin Engelhardt wrote on the San Gabriel Mission (published in 1927) and the San Fernando Mission (1922) but they do not provide any details. The San Gabriel Mission book does have a map of the "Old California Missions" but it is very general.
There was a dirt road connecting the missions, presidios and ultimately major towns, called El Camino Real, or the Kings Highway. The exact layout of the road varied over time as natural disasters (particularly changes in the direction of rivers, earthquakes etc.) forced a new routing, but particularly as missions were added to the chain.