You will have 2 routes you can travel through kochi alleppy and reach kanyakumari or you can enter tamilnadu ave go through madu dai. The second route has very good roads for riding if you find this answer helpful please leave an upvote to this reply.
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You have to travel via Trivandrum. As I took that route. So our trip started from Kochi and from there Munnar, Thekkady, Alleppey, Trivandrum and finally Kanyakumari.
Munnar-thekkadi-allepley (Marari on the other side quite off beat and amazing and 4km from alleppey)- kollam (Munroe island may be, very beautiful)- varkala-kovalam or trivandrum- kanyakumari