Where to obtain metabolic pathway datasets
KEGG is definitely your choice!

It is possible to get the whole list(s) of pathways: http://rest.kegg.jp/list/pathway (reference pathways) or http://rest.kegg.jp/list/pathway/hsa (for human).

Then you can use these IDs to retrieve information about individual pathways: http://rest.kegg.jp/get/hsa00053 (GeneBank-like format), http://rest.kegg.jp/get/hsa00053/image (image), http://rest.kegg.jp/get/hsa00053/kgml (KEGG xml). It will take a while, of course, and I'd recommend not to overload their server and make delays between queries. With the GeneBank-like format batch querying is also possible: http://rest.kegg.jp/get/hsa00053+hsa00061

This should likely be a one-time or at least rare operation, because of the load on their server (and I don't think they make big changes very often). You can retrieve info for new pathways, once they appear in http://rest.kegg.jp/list/pathway/...