Speaking/clapping rhythms - ain't no shame there! Sometimes in choir with complex music (e.g. Bernstein's Chichester Psalms - here ) you need to sort out the rhythm before you start worrying about notes or words ;}All good advice. Walking rhythms is great. Getting the pulse or feel is important so slowing it down and really understanding note lengths is essential. You could get the ABRSM workbook Improve Your Sight Reading, starting with grades 1- 3. There are graded exercises for playing and clapping/saying note lengths and rhythms. I know it sounds cheesy but if you are not sure, clapping/saying really does help get the basics anchored. It covers minims, crotchets, quavers, (sorry, half notes, quarter notes notes, eighth notes for USA friends), dotted notes, tied notes, different time sigs. Then you can move on to the higher levels.
When you are walking, run rhythms through your head. Eighths against triplets for example with each step being a beat of a measure, 1 and 2 and 3 and a 4 and a. Then build in complexity. This can be especially helpful with normalizing off beats and grace notes/gliss' in your head.